A new study by the Salk Institute shows that a cluster of neurons in our spinal cords function as a “mini-brain” to help us walk without constant monitoring by the brain.
In a paper published last week in the journal Cell, Salk scientists mapped the neural circuitry of the spinal cord that processes the sense of light touch. This circuitry allows the body to reflexively make small adjustments to foot position and balance using light touch sensors in the feet.
The study, conducted in mice, provides the first detailed blueprint for a spinal circuit that serves as control center for integrating motor commands from the brain with sensory information from the limbs.
A better understanding of these circuits should eventually aid in developing therapies for spinal cord injury and diseases that affect motor skills and balance, as well as the means to prevent falls for the elderly.
“When we stand and walk, touch sensors on the soles of our feet detect subtle changes in pressure and movement. These sensors send signals to our spinal cord and then to the brain,” says Martyn Goulding, a Salk professor and senior author on the paper.
“Our study opens what was essentially a black box, as up until now we didn’t know how these signals are encoded or processed in the spinal cord. Moreover, it was unclear how this touch information was merged with other sensory information to control movement and posture.”
Using cutting-edge imaging techniques, the researchers traced nerve fibers that carry signals from the touch sensors in the feet to their connections in the spinal cord. They found that these sensory fibers connect in the spinal cord with a group of neurons known as RORα neurons, named for a specific type of molecular receptor found in the nucleus of these cells.
The RORα neurons in turn are connected by neurons in the motor region of brain, suggesting they might serve as a critical link between the brain and the feet
Another important characteristic of the RORα neurons is that they don’t just receive signals from the brain and the light touch sensors, but also directly connect with neurons in the ventral spinal cord that control movement. Thus, they are at the center of a “mini-brain” in the spinal cord that integrates signals from the brain with sensory signals to make sure the limbs move correctly.
“We think these neurons are responsible for combining all of this information to tell the feet how to move,” says Steeve Bourane, a postdoctoral researcher in Goulding’s lab and first author on the new paper. “If you stand on a slippery surface for a long time, you’ll notice your calf muscles get stiff, but you may not have noticed you were using them. Your body is on autopilot, constantly making subtle corrections while freeing you to attend to other higher-level tasks.”
The team’s study represents the beginning of a new wave of research that promises to provide precise and comprehensive explanations for how the nervous system encodes and integrates sensory information to generate both conscious and unconscious movement.
The Salk Institute for Biological Studies in La Jolla is one of the world’s preeminent basic research institutions. Founded in 1960 by polio vaccine pioneer Jonas Salk, the institute is an independent nonprofit organization and architectural landmark.